Lec. (2) CROWN AND BRIDGE أ.م. هدى عباس عبد اهلل Patient selection and examination A thorough diagnosis must first be made of the patient's dental condition, considering both hard and soft tissues. this must correlated with the individuals overall physical health and psychological needs. There are five elements to a good diagnostic workup in preparation for fixed prosthodontic treatment. 1. History 2. TMJ/occlusion evaluation 3. Intraoral examination 4. Diagnostic casts 5. Radiograph History: it is important that a good history be taken before the initiation of treatment to determine if any special precautions are necessary. TMJ/occlusion evaluation: the patients occlusion must be evaluated to determine if it is healthy enough to allow the fabrication of such restoration. If the occlusion is within the normal limits, then all the treatment should be designed to maintain that occlusal relationship. Intraoral examination; when the patient's mouth is examined, there are several observations to be made. first of these is the oral hygiene. Then check attached gingival around all teeth, particularly around teeth to be restored with crowns. The presence or absence of inflammation should be noted, the presence and amount of tooth mobility should be recorded. Examine edentulous ridges and note the relationship of spaces if there are more than one. condition of abutment teeth, presence and location of caries, previous restorations and prostheses examined carefully. Finally occlusion should be evaluated. Diagnostic casts; an integral part of the diagnostic procedures necessary to give the dentist as complete a perspective as possible of the patient's dental needs. Radiographs; it is the final aspect of the diagnostic procedure, providing the dentist with information to help correlate all of the facts that have been collected in listening to the patient, examining the mouth, and evaluating the diagnostic casts. x-ray should be evaluated carefully.
Principles of tooth preparation Objective of tooth preparation:- 1. To provide esthetic أ.م. هدى عباس عبد اهلل 2. Maintain the normal size of the tooth after placement of the prosthesis 3. Eliminate undercuts from the axial surface of the tooth 4. Provide enough space for the crown restoration to withstand the force of mastication, this space depends on the material used so the metal material needs little space while the plastic material needs more space. Principles of tooth preparation;- 1. Preservation of tooth structure. 2. Retention and resistance. 3. Structural durability. 4. Marginal integrity. 5. Preservation of periodontium 1. Preservation of the tooth structure The preparation of the tooth must be conservative the minimal amount of the tooth structure must be removed, excessive amount of the tooth structure in addition to be destructive phenomenon it has many harmful effects;- 1. Excessive reduction lead to thermal hypersensitivity, pulpal inflammation and necrosis may result from approaching to the pulp closely. 2. The tooth might be over tapered or shortened and this might affect the retention and resistance of the prepared tooth. 2. Retention and resistance:- Retention prevents removal of the restoration along the path of insertion or long axis of the tooth preparation.
جامعة تكريت كلية Resistance prevents dislodgement of the restoration by forces directed in an apical or oblique direction and prevents any movement of the restoration under occlusal forces. Retention and resistance are interrelated and often inseparable qualities. Path of insertion:- an imaginary line along which the restoration can be inserted and removed without causing lateral force on the abutment. the crown restoration should have a single path of insertion to be retentive. Most of the time the path of insertion of crown is parallel to the long axis of the tooth,except in 3/4 crown for anterior teeth the path of insertion should be parallel to the incisal 2/3 of the tooth crown. Retention is improved by limiting the number of paths along which a restoration can be removed from the tooth preparation.
جامعة تكريت كلية Factor affecting retention and resistance 1. Taper of the preparation 2. Surface area of the preparation 3. Length and height of the preparation 4. Diameter of the tooth 5. Texture of the preparation 6. Accessory mean 1. Taper of the preparation Convergence angle; is angle formed between opposing axial walls, retention of the crown depend on this angle, 5-6 degree convergence angle is mostly used to provide the needed retention. Theoretically, the more nearly parallel the opposing walls of a preparation, the greater is the retention. Resistance;-the more parallel the axial walls the more will be the resistance of crown restoration, The walls of short wide preparation must be kept nearly parallel to achieve adequate resistance form.
2. Surface area of the preparation Increasing the surface area increase retention. Factors that influence surface area;- أ.م. هدى عباس عبد اهلل a) Size of the tooth Large size of the tooth more surface area of the preparation more retention it will be obtain, crown on molar tooth more retentive it will be than on premolar b) Extend of coverage by restoration; The more area that will be cover by the crown restoration the more will be the retention, thus full metal crown on molar is more retentive than 3/4 crown on the same tooth. c) Accessory feature. Such as boxes, groove and pinhole. 3. Length( height) of the preparation;- Increase the length increase retention and resistance and vice versa. 4. Diameter of the tooth; Under some circumstances crown on narrow tooth can have greater resistance to tipping than one on the wider tooth. 5. Texture of the preparation; Might effect on the retention of cast crown. Smooth surfaces less retentive than rough (mechanical interlocking). 6. Accessary mean;- grooves, pinholes and boxes lead to increase retention. 3. Structural durability Restoration must contain bulk of material that is adequate to withstand the forces of occlusion. This bulk must be confined to
the space created by the tooth preparation. Only this way the normal. occlusion will be harmonious and axial contours Occlusal reduction. Enough tooth surface must be removed from occlusal surface so that resistance can built back to ideal occlusion & thick enough to prevent wearing or distortion (1-1.5mm). Occlusal clearance:-is the space between the occlusal surface of the prepared tooth and that of apposing tooth. It should be evaluated in centric and eccentric relation. Flat occlusal Reduction; lead to thin metal this will lead to perforation of the crown restoration in the future. Occlusal Reduction; must reflect the geometric inclined planes of occlusal surface. Avoid creating steep planes with sharp angles,because it lead to stress. Avoid excessive lowering entire occlusal surface might lead to tooth structure destruction. Functional cusp bevel;- A wide bevel placed on the functional cusp (lingual inclines of the maxillary lingual cusps and the buccal inclines of
mandibular buccal cusps) provides space for an adequate bulk of contact. metal in an area of heavy occlusal Axial reduction;- Plays an important role in securing space for an adequate thickness of the restorative material. Inadequate axial reduction will have thin walls subject to distortion or result in over contouring the axial surface which could lead to periodontal problems 4. Preservation of periodontal tissue Margin placement (finishing line) should be placed supergingivally. Finishing line can be placed either; 1. Supragingival; Placing the margin above the gingival tissue for the following reasons; a. can easily prepared and finished b. to provide good vision for the dentist during preparation. c. impression can be easily made d. the patient can keep the area clean easily e. most of the time such position is situated on hard enamel. f. less destructive. 2. Subgingival; Placing the margin below the gingival tissue for following reasons;-
a. when the esthetic is factor. b. when we need extra retention. c. when we have caries or filling at the area of finish line. 3. placing the margin within the level of the gingiva 5. Marginal integrity The restoration margin should closely adapt to the cavosurface finish line (CSL) of the preparation to survive in a biological environment in the oral cavity. Restoration margins should have the following requirements;- A. They must fit as closely as possible against the finishing line of preparation. B. They must have sufficient strength C. Whenever possible they should be placed in an area where the dentist can finish and clean them properly. Types of finishing line:- 1.Knife egde(featheredge) 2.Chamfer and heavy chamfer 3.Shoulder finishing line A. Butt shoulder B. Shoulder with bevel C. Radial shoulder 1. Knife edge (featheredge) use pointed end fissure bur to provide this type of margin, it s the most conservative type of finishing line, margin is weak it form >135 cavosurface line angle
Advantages; 1.it's the most conservative type of finishing line. 2.it's easy to prepared. 3.it's burnishable type of finishing line. Disadvantages; 1.difficult to identified 2.thin margin that difficult to accurately wax and cast 3.more susceptible to distortion. Its mainly used for; Full metal crown, lingual and proximal surfaces of full veneer crown, 3/4 crown and post crown. 2. Chamfer finishing line Cut is made deep, it form 130-160 CSA(round end diamond) 1. Well define f.l. 2. Provide more room for a bulk of metal 3. It is slightly more difficult to burnish 4. It indicated for area to be cover by gold(metal) margin It is mainly used for, full metal crown, lingual and proximal surfaces of full veneer crown,3/4 crown and post crown Round end tapered fissure bur is used to obtain this margin. Heavy chamfer; It provide 90 C.S.L. with a large radius internal angle, it provide better support for ceramic crown. 3. Shoulder f.l.;
a. Butt shoulder; it's the least conservative type of f.l. because we need to removed excessive amount of tooth structure to obtian it. axial walls meet the f. l. at right angle90. Advantages;- 1.Britle material need sufficient thick section to withstand occlusal force 2.This thickness is necessary to provide more translucent porcelain to simulate the appearance of the natural tooth. 3. wide ledge provide resistance to occlissal forces&minimize stress that to fracture. 4.it provide the space for healthy rest. Contour&esthetic Radial shoulder;- it is shoulder finishing line with rounded internal line angles, this will reduce the shoulder slightly & minimize stress concentration on tooth structure. Shoulder with bevel f. l. It is modification of s.f.l. by adding bevel to the shoulder to produce a f.l. like knife edge, the bevel is 45deg. Angle. Objectives; 1. The bevel provides a burnishable margin for the metal that extend subgingivally.(the thinner it is the more adaptable to the tooth surface) 2. Provide enough space for shape and contour 3. To reduce marginal discrepancies. 4. Removing unsupported tooth structure 5. It indicated when we use a combination of metal with facing material (acrylic or porcelain) such as full veneer crown. Shoulder with bevel is
used on the labial surface of full veneer crown and is recommended for extremely short walls. Factors affecting selection:- 1. Type of the restoration 2. Material used in construction 3. The amount of occlussal force(stress) the restoration will bear